A plasma processing apparatus generates a plasma in a process chamber for treating a workpiece supported by a platen in the process chamber. A plasma processing apparatus may include, but not be limited to, doping systems, etching systems, and deposition systems. The plasma is generally a quasi-neutral collection of ions (usually having a positive charge) and electrons (having a negative charge). The plasma has an electric field of about 0 volts per centimeter in the bulk of the plasma. In some plasma processing apparatus, ions from the plasma are attracted towards a workpiece. In a plasma doping apparatus, ions may be attracted with sufficient energy to be implanted into the physical structure of the workpiece, e.g., a semiconductor substrate in one instance.
The plasma is bounded by a region proximate the workpiece generally referred to as a plasma sheath. The plasma sheath is a region that has fewer electrons than the plasma. The light emission from this plasma sheath is less intense than the plasma since fewer electrons are present and hence few excitation-relaxation collisions occur. Hence, the plasma sheath is sometimes referred to as “dark space.”
Turning to FIG. 1, a cross sectional view of portions of a known plasma processing apparatus is illustrated where a plasma 140 has a plasma sheath 142 adjacent to a front surface of a workpiece 138 to be treated. The front surface of the workpiece 138 defines a plane 151, and the workpiece 138 is supported by a platen 134. The boundary 141 between the plasma 140 and the plasma sheath 142 is parallel to the plane 151. Ions 102 from the plasma 140 may be attracted across the plasma sheath 142 towards the workpiece 138. Accordingly, the ions 102 that are accelerated towards the workpiece 138 generally strike the workpiece 138 at about a 0° angle of incidence relative to the plane 151 (e.g., perpendicular to the plane 151). There can be a small angular spread of the angle of incidence of less than about 3°. In addition, by controlling plasma process parameters such as gas pressure within a process chamber, the angular spread may be increased up to about 5°.
A drawback with conventional plasma processing is the lack of angular spread control of the ions 102. As structures on the workpiece become smaller and as three dimensional structures become more common (e.g., trench capacitors, vertical channel transistors such as FinFETs) it would be beneficial to have greater angle control. For example, a trench 144 having an exaggerated size for clarity of illustration is shown in FIG. 1. With ions 102 being directed at about a 0° angle of incidence or an even angular spread up to 5°, it can be difficult to uniformly treat the sidewalls 147 of the trench 144.
Accordingly, there is a need for a plasma processing apparatus which overcomes the above-described inadequacies and shortcomings.